An implantable medical device (IMD), such as a pacemaker and/or implantable cardioverter-defibrillator (ICD), regulates or synchronizes the beating of the heart with electrical impulses, delivered by electrodes contacting the heart muscles. Some IMDs include a number of different sensors and logic allowing them to monitor the rate and rhythm of the heart as well as to measure various cardiac surrogates that provide information on the operation of the heart.
One of the primary purposes of such IMDs is to maintain an adequate heart rate in chronotropic incompetent patients. Chronotropic incompetence is generally considered the inability of a patient to achieve an adequate heart rate in response to physiological need, such as during exercise. Such chronotropic incompetence may be due to the heart's natural pacemaker being inadequate, problems with the heart's electrical conduction system, age, medication, and the like. However, treatment of chronotropic deficiencies using an IMD is not always a simple matter of firing off the heart to beat at a certain time. It often includes the complex synchronization of the individual movements and processes that make up each stage of a typical heart beat. As measurements are made and analyzed by the IMD, electrical therapies may be delivered when the performance or synchronization of the heart varies from some pre-defined measurement of normal operation.
Because each patient's heart and circulatory system is different and may have different physiological responses over time even within its own operation, programmable fixed-rate systems generally do not provide optimal or sometimes even adequate treatment to patients. In response, rate responsive pacemaker systems have been developed which typically include some means or methods for monitoring at least one patient-specific variable. Based on this patient-specific variable, the IMD can determine an indicated pacing rate as a function of the sensed pacing variable. This rate responsive system, referred to herein as rate programming control (RPC), allows the IMD to optimally control pacing rate in terms of the patient's condition. Thus, such RPC functionality generally provides an improved response to the patient's physiological needs, as compared to programmable fixed rate pacemakers.
One of the ultimate goals of such IMDs is to increase a patient's cardiac output in order to meet the patient's physiological needs. The presumption for such treatment is that increasing the heart rate will boost the cardiac output by increasing the stroke volume. Stroke volume is the amount of blood pumped by the ventricle during each beat cycle. It is equal to the difference between the end diastolic volume (EDV) (the volume of blood in the ventricle at its most full) and the end systolic volume (ESV) (the volume of blood remaining in the ventricle after it completes contraction). Under normal physiological conditions, increasing heart rate will naturally increase the level of ventricular contractile force (i.e., contractility). This force-frequency relationship is known as the Treppe effect. However, increasing the heart rate without considering peripheral resistance may cause ischemia/infarction or atrial fibrillation.